Information recording medium

ABSTRACT

An information recording medium having a substrate, a recording layer on which information is recorded in accordance with a change of atomic arrangement caused by irradiation of light, a first reflective layer, and a second reflective layer. The second reflective layer is formed on the first reflective layer. The first reflective layer includes an Al alloy as a main component and the second reflective layer includes an Ag alloy as a main component.

CROSS REFERENCE TO RELATED APPLICATION

This is a continuation of U.S. application Ser. No. 09/832,137, filedApr. 11, 2001, now U.S. Pat. No. 6,436,504, which is a continuation ofU.S. application Ser. No. 09/380,302, filed Aug. 30, 1999, now U.S. Pat.No. 6,231,945, which is a national stage entry of PCT/JP98/04019 filedSep. 8, 1998, the subject matter of which is incorporated by referenceherein.

TECHNICAL FIELD

The present invention relates to an information recording medium usedfor an optical disk.

BACKGROUND ART

Various principles are known for recording information in a thin film(recording film) by radiating a laser beam on it. Among them, a methodutilizing the change of the atomic arrangement by the radiation of thelaser beam such as the phase transition (also called the phase change)of the film material or the photo darkening is not substantiallyaccompanied by the deformation of the thin film, and therefore has theadvantage that an information recording medium of a two-side diskstructure can be obtained by attaching two disk members directly to eachother.

Generally, these information recording media are configured of asubstrate, and a protective layer, a recording film of GeSbTe group,etc., a protective layer and a reflective layer formed on the substrate,and the reflectance is higher in crystal state than in amorphous state.Therefore, the absorption of a recording film is larger for amorphus.The recording mark portion in amorphous state is liable to increase intemperature more easily than the crystal. In the case where an overwriteoperation is performed in this state, therefore, a newly recorded markis increased excessively, thereby causing the reproduction signal to bedistorted.

In order to prevent this inconvenience, an effort has been made toincrease the absorption coefficient of the recording film in crystalstate as compared with that of the recording film in amorphous state.For example, reference 1 “Yamada and three others, Shingaku GihoMR92-71, CPM92-148, December 1992, p. 37” describes a structure formedwith an Au reflective layer as thin as 10 nm to reverse the absorptioncoefficient.

Also, reference 2 “Okada and six others, Shingaku Giho MR93-53,CPM93-105, December 1993, p. 1” describes a structure in which Si of 65nm is used for the reflective layer thereby to reverse the absorptioncoefficient.

In this specification, the term “phase change” is defined to include notonly the phase change between crystal and amorphus but also betweenmelting (change to liquid phase) and recrystallization and betweencrystal states.

DISCLOSURE OF INVENTION

In all the conventional information recording media used as ahigh-density rewritable information recording media of phase transitiontype using a mark edge recording, the erase characteristic is improvedby reversing the absorption coefficient (the absorption coefficient ofamorphus is lower than that of crystal). These media use a material witha thin reflective layer or a material of a reflective layer throughwhich light is transmitted or has a structure allowing light to transmittherethrough. This method poses the problem that each absorptioncoefficient is smaller than that of the normal disk with the absorptioncoefficient not reversed, resulting in a small recording sensitivity.The laser used for recording having a wavelength shorter than about 680nm is still low in output, and if the linear speed is increased forincreasing the transfer rate, the recording sensitivity tends todeteriorate. Therefore a medium of high recording sensitivity isrequired.

Further, the use of a thin material of a low heat conductivity for thereflective layer poses the problem that heat generated at the time ofrecording is not easily diffused often causing an increased jitter aftera multiplicity of overwrite cycles.

Accordingly, an object of the present invention is to solve theseproblems and to provide an information recording medium which has asuperior recording/reproduction characteristic without increasing thejitter as compared with the prior art even after the overwrite cycle forhigh-density recording and reproduction.

Means for solving the problems will be described below.

(1) There is provided an information recording medium comprising aninformation recording thin film as a recording layer formed on asubstrate for recording and/or reproducing information by the change inatomic arrangement caused by the radiation of light, and at least oneprotective layer, wherein the protective layer and the recording layerare formed in that order from the light incidence side, followed bybeing formed with at least one absorption control layer.

(2) There is provided an information recording medium as described in(1), characterized in that the thickness of the absorption control layeris in the range of not less than 10 nm but not more than 50 nm. Thethickness of not less than 10 nm but not more than 40 nm is morepreferable.

(3) There is provided an information recording medium as described in(1), wherein at least 95% of the total number of atoms of the absorptioncontrol layer is composed of a mixture or a compound of a dielectricmaterial and a metal element.

(4) There is provided an information recording medium as described in(1), wherein the absorption control layer is made of a material having n(refractive index) of not less than 1.2 but not more than 6, and k(extinction coefficient) not less than 0.5 but not more than 3.3. If nis not less than 1.8 but not more than 5.5, and k is not less than 0.8but not more than 3, it is more preferable.

(5) There is provided an information recording medium as described in(1), wherein the absorption control layer is made of a material having amelting point of not less than 600° C.

(6) There is provided an information recording medium as described in(1), characterized in that in the case where information is recorded onthe recording film, the reflectance in amorphous state is lower thanthat in crystal state, the reflectance in amorphous state is lower thanthe reflectance in crystal state, and the mark size with the shortestmark recorded on a material in amorphous state is equal to or smallerthan the mark size with the shortest mark recorded on a material incrystal state under the same conditions.

(7) There is provided an information recording medium as described in(1), wherein at least a heat diffusion layer is formed between thesubstrate and the protective layer.

(8) There is provided an information recording medium comprising aninformation recording thin film as a recording layer formed on asubstrate for recording and/or reproducing information by the change inatomic arrangement caused by the radiation of light, at least aprotective layer, at least a heat diffusion layer and at least a heatdiffusion layer, characterized in that the heat diffusion layer, theprotective layer and the recording layer are formed in that order fromthe light incidence side, followed by being formed with at least onereflective layer.

(9) There is provided an information recording medium as described inany one of (7) and (8), characterized in that at least 90% of the totalnumber of atoms of the heat diffusion layer is composed of Al-0.

(10) There is provided an information recording medium as described inany one of (7) and (8), characterized in that the heat diffusion layerhas a layer in which at least 90% of the total number of atoms has acomposition similar to any one of (SiO₂), (Al₂O₃), (Ta₂O₅),(Al₂O₃)—(SiO₂), (Ta₂O₅)—(SiO₂), (Al₂O₃) —(Ta₂O₅) and(Al₂O₃)—(SiO₂)—(Ta₂O₅) or a mixture composition thereof.

(11) There is provided an information recording medium as described inany one of (7) to (8), wherein the heat diffusion layer has a layer inwhich at least 90% of the total number of atoms has a compositionsimilar to any one of Be—O, B—N, Si—C and Mg—O or a mixture compositionthereof.

(12) There is provided an information recording medium as described in(1), characterized by having a structure wherein a reflection layercomposed of at least one layer of a Cu alloy, an Al alloy and an Aualloy is formed on the absorption control layer.

(13) There is provided an information recording medium as described in(1) or (8), characterized in that at least one surface protect layer isformed in the boundary of the recording film.

(14) There is provided an information recording medium as described in(1) or (8), characterized in that the recording film satisfies therelation

Ge_(x-w)Sb_(y)Te_(z)M_(w)

where 0.10≦x ≦0.26, 0.18≦y ≦0.33, 0.52≦z≦0.60, w≦0.06 and x+y+z=1, and Mis any one of Na, Mg, Al, P, S, Cl, L, Ca, Sc, Zn, Ga, As, Se, Br, Rb,Sr, Y, Zr, Nb, Ru, Rh, Cd, In, Sn, I, Cs, Ba, La, Hf, Ta, Re, Os, Ir,Hg, Tl, Pb, Th, U, Ag, Cr, W, Mo, Pt, Co, Ni, Pd, Si, Au, Cu, V, Mn, Fe,Ti and Bi.

(15) There is provided an information recording medium as described in(1) or (8), characterized in that the protective layer is made of alayer containing at least 80 mol % of ZnS.

(16) In the case where the absorption control layer is made ofMo—(SiO₂), the MO amount represents preferably not less than 42 mol % ofall the components. The figure of not less than 61 mol % but not morethan 90 mol % is more desirable.

The use of Cr, W, Fe, Sb, C, Zn, Mn, Ti, Co, Ge, Pt, Ni, Nb, Pd, Be orTa as a material replacing Mo in the Mo—(SiO₂) film of the absorptioncontrol layer has produced a similar result. Among these elements, Mo,Cr and W are more preferable as they have a high melting point. Also, Pdand Pt are not very reactive with other layers and the resultingincreased possible number of overwrite cycles makes these elements morepreferable. When Ni, Co or Ti is used, on the other hand, an inexpensivetarget can be used as compared with other materials and the totalproduction cost can be reduced.

Materials which may be used in place of SiO₂ in the Mo—(SiO₂) film usedfor the absorption control layer include oxides including SiO, Al₂O₃,BeO, Bi₂O₃, CoO, CaO, Cr₂O₃, CeO₂, Cu₂O, CuO, CdO, Dy₂O₃, FeO, Fe₂O₃,Fe₃O₄, GeO, GeO₂, HfO₂, In₂O₃, La₂O₃, MgO, MnO, MoO₂, MoO₃, NbO, NbO₂,NiO, PbO, PdO, SnO, SnO₂, Sc₂O₃, SrO, ThO₂, TiO₂, Ti₂O₃, TiO, Ta₂O₅,TeO₂, VO, V₂O₃, VO₂, WO₂, WO₃, Y₂O₃ and ZrO₂, nitrides including AlN,BN, CrN, Cr₂N, GeN, HfN, Si₃N₄, Al—Si—N group material (such as AlSiN₂),Si—N group material, Si—O—N group material, TaN, TiN and ZrN, sulfidesincluding ZnS, Sb₂S₃, CdS, In₂S₃, Ga₂Se₃, GeS, SnS₂, PbS, Bi₂S₃, SrS,MgS, CrS, CeS and TaS₄, selenides including SnSe₂, Sb₂Se₃, CdSe, ZnSe,In₂Se₃, Ga₂Se₃, GeSe, GeSe₂, SnSe, PbSe and Bi₂Se₃, fluorides includingCeF₃, MgF₂, CaF₂, TiF₃, NiF₃, FeF₂ and FeF₃, Si, Ge, borides includingTiB₂, B₄C, B, CrB, HfB₂, TiB₂ and WB, carbides including C, Cr₃C₂,Cr₂₃C₆, Cr₇C₃, Fe₃C, Mo₂C, WC, W₂C, HfC, TaC and CaC₂, or a materialhaving a composition similar to any of the materials described above ora mixture thereof.

Among these materials, the use of SiO₂, Ta₂O₅ or Y₂O₃-ZrO₂ makes itpossible to use a target less expensive than when using other materials,and therefore can reduce the whole cost of production.

Al₂O₃ is high in heat conductivity. Therefore, a disk having a structurelacking the first reflective layer and/or the second reflective layerdeteriorates the rewrite characteristic to lesser degree than when usingother materials.

Also, in the case where the absorption control layer contains impuritieselements not more than 5 atomic % of the components thereof, it candesirably reduce the deterioration of the rewrite characteristic. Thecontent of not more than 2 atomic % is more preferable.

(17) A preferable material of the upper surface protect layer and thelower surface protect layer is SiO₂, Al₂O₃ or a mixture of Al₂O₃ andSiO₂. In the case where 70 mol % or more of SiO₂ or Al₂O₃ is contained,the crystallization rate is increased and at 18 m/s that is the rateabout twice as high as in the absence of the surface protect layer, theerasure ratio reaches 25 dB or more.

The next preferable choice is Ta₂O₅ or a mixture between Ta₂O₅ and SiO₂or Al₂O₃. The second next preferable choice is ZrO₂—Y₂O₃, SiO₂ or amixture of ZrO₂—Y₂O₃ or SiO₂ with Al₂O₃ or Ta₂O₅. Among these materials,Al₂O₃ is more preferable as it can suppress the variations of thereflectance level to 5% or less and can reduce the jitter after amultiplicity of overwrite cycles. The materials CoO, Cr₂O₃ and NiO arealso more preferable as a uniform crystal grain size is obtained at thetime of initial crystallization and the jitter is increased to a lesserdegree in the initial stage of overwrite cycle.

Also, nitrides such as AlN, BN, CrN, Cr₂N, GeN, HfN, Si₃N₄, Al—Si—Ngroup material (such as AlSiN₂), Si—N group material, Si—O—N groupmaterial, TaN, TiN and ZrN are more preferable as they increase theadhesion and deteriorate the information recording medium to a lesserdegree under external shocks. A material of the recording filmcontaining nitrogen or a material having a similar composition can alsoimprove the adhesion.

In addition, oxides such as BeO, Bi₂O₃, CeO₂, Cu₂O, CuO, CdO, Dy₂O₃,FeO, Fe₂O₃, Fe₃O₄, GeO, GeO₂, HfO₂, In₂O₃, La₂O₃, MgO, MnO, MoO₂, MoO₃,NbO, NbO₂, PbO, PdO, SnO, SnO₂, Sc₂O₃, SrO, ThO₂, TiO₂, Ti₂O₃, TiO,TeO₂, VO, V₂O₃, VO₂, WO₂ and WO₃ or carbides such as C, Cr₃C₂, Cr₂₃C₆,Cr₇C₃, Fe₃C, MO₂C, WC, W₂C, HfC, TaC and CaC₂ or materials having asimilar composition can also be used.

As another alternative, any mixture of these materials is usable.

The upper surface protect layer, the lower surface protect layer, andthe replacement materials of the upper surface protect layer and thelower surface protect layer preferably represent 90% or more of thetotal number of atoms of the respective surface protect layer. In thecase where impurities other than the materials described above reach tenatomic % or more, the possible number of overwrite cycles is reduced by50% or more, or otherwise the rewrite characteristic is deteriorated.

In the absence of the upper surface protect layer, the reflective layermaterial diffuses into the recording film and the remanence increases,the reduction in the reflectance level after 100 thousand overwritecycles can be suppressed to as small as 5% or less. A change inreflectance level causes the offset of the reproduction signal level,and adds the offset jitter for an increased jitter. Thus, the variationof the reflectance level is preferably as small as possible.

Further, for the modulation degree to be maintained at 43% or more, thefigure of not more than 12 nm is preferable. For the figure of 5 nm orless, the modulation degree of 47% or more can be secured. A film ofuniform thickness can be formed when the thickness is not less thanabout 2 nm. In the case where the thickness of the upper surface protectlayer is 2 to 12 nm, therefore, the recording/reproductioncharacteristic is desirably improved.

In the absence of the lower surface protect layer, the protective layermaterial diffuses into the recording film for an increased remanence, sothat the jitter increases beyond 6% after 100 thousand overwrite cycles.Further, for maintaining the modulation degree at 43% or more, thethickness is desirably maintained at 25 nm or less. The thickness of notless than 5 nm but not more than to 10 nm can secure the modulationdegree of 47% or more. In view of the fact that a uniform film is formedfor the thickness of about 2 nm or more, the recording/reproductioncharacteristic is desirably improved when the thickness of the lowersurface protect layer is 2 to 25 nm.

(18) Materials of the protective layer include any one of ZnS, Si—Ngroup material, Si—O—N group material, oxides such as SiO₂, SiO, TiO₂,Al₂O₃, Y₂O₃, CeO₂, La₂O₃, In₂O₃, GeO, GeO₂, PbO, SnO, SnO₂, BeO, Bi₂O₃,TeO₂, WO₂, WO₃, Sc₂O₃, Ta₂O₅, ZrO₂, Cu₂O and MgO, nitrides such as TaN,AlN, BN, Si₃N₄, GeN, Al—Si—N group material (such as AlSiN₂), sulfidessuch as ZnS, Sb₂S₃, CdS, In₂S₃, Ga₂S₃, GeS, SnS₂, PbS and Bi₂S₃,selenides such as SnSe₂, Sb₂Se₃, CdSe, ZnSe, In₂Se₃, Ga₂Se₃, GeSe,GeSe₂, SnSe, PbSe and Bi₂Se₃, fluorides such as CeF₃, MgF₂ and CaF₂, orSi, Ge, TiB₂, B₄C, B, C or materials having a similar composition to thematerials described above. Also, a layer of a mixtures a multi-layer ofthese materials including ZnS—SiO₂ and ZnS—Al₂O₃ may be used. Amongthese materials, ZnS has a large n and can maintain a large modulationdegree. In the case of a mixture containing 60 mol % or more of thismaterial, the large n of ZnS and the superior chemical stability of theoxide have a combined effect. Further, ZnS has a large sputter rate, sothat when ZnS represents 80 mol % or more, the film-producing time canbe shortened. Other sulfides and selenides can also produce similarcharacteristics.

The element ratio in these compounds, i.e. the ratio between a metalelement and oxygen element for oxides and the ratio between a metalelement and a sulfide element for sulfides, for example, is preferably 2to 3 or thereabouts for Al₂O₃, Y₂O₃ and La₂O₃, 1 to 2 or thereabouts forSiO₂, ZrO₂ and GeO₂, 2 to 5 or thereabouts for Ta₂O₅ and 1 to 1 orthereabouts for ZnS. Even a ratio departing from the ratios specifiedabove can product a similar effect. In the case where the ratio is notan integral one described above, for example, the deviation of the ratiobetween Al and O in Al—O is preferably not more than ±10 atomic % interms of Al amount from Al₂O₃, the deviation of the ratio between Si andO in Si—O is preferably not more than ±10 atomic % in terms of Si amountfrom SiO₂. In this way, the deviation of not more than 10 atomic % isdesirable. A deviation of not less than 10 atomic % would change theoptical characteristic and the modulation degree is reduced by 10% ormore.

The protective layer and the replacement material of the protectivematerial preferably represents at least 90% of the total number of atomsof the respective protective layer. In the case where impurities otherthan these materials increase to 10 atomic % or more, the possiblenumber of overwrite cycles is reduced to one half or less or otherwisethe rewrite characteristic is deteriorated.

The thickness of the protective layer is desirably 20 to 70 nm, whichcan increase the modulation degree for recording to as high as 43% ormore, and more preferably, the thickness of the protective layer is 35to 60 nm.

(19) The preferable materials of the heat diffusion layer are Al₂O₃,MgO, BeO, SiC, BN, B₄C large in heat conductivity. Also, Ta₂O₃, SiO₂,Al₂O₃ and mixtures thereof have an inexpensive target and the productioncost thereof is desirably low. On the other hand, ThO₂, TiO₂, AlN andTiN are desirable for their ease to form into a film.

Other preferable materials than those described above have a heatconductivity larger than the substrate material and an absorptioncoefficient k smaller than 0.5.

A large heat conductivity can suppress the damage to the substratesurface by heat at the time of recording, and therefore the jitter canbe suppressed to a low level after 100 thousand overwrite cycles. Also,a small k can suppress the reduction of modulation degree to a smalllevel.

The heat diffusion layer and the replacement materials of the heatdiffusion layer are desirably not less than 90% of the total number ofatoms of each protective layer. In the case where the impurities otherthan the materials described above reaches 10 atomic % or more, thepossible number of overwrite cycles is reduced to one half or less orotherwise the rewrite characteristic is deteriorated.

The thickness of the heat diffusion layer is preferably 10 to 50 nm ormore preferably 20 to 40 nm.

(20) The preferable material of the first reflective layer is Al—Cr,Al—Ti, Al—Ag or the like containing an Al alloy which can reduce thejitter to a low level at the time of overwrite operation.

The characteristic for a multiplicity of overwrite cycles has been foundto be improved when the contents of the element other than Al in the Alalloy reaches the range of not less than 5 atomic % but not more than 30atomic %. A similar characteristic is obtained also from the Al alloyother than those described above.

A layer may be used which is composed of any one of the element unit Au,Ag, Cu, Ni, Fe, Co, Cr, Ti, Pd, Pt, W, Ta, Mo, Sb, Bi, Dy, Cd, Mn, Mg orV or an alloy containing any one of these materials as a main componentsuch as an Au alloy, Ag alloy, Cu alloy, Pd alloy, Pt alloy, Sb—Bi, SUS,Ni—Cr or alloys between these alloys. In this way, the first reflectivelayer is composed of a metal element, a metalloid element, an alloy or amixture thereof.

Among these materials, such materials as Cu alloy, Al alloy or Au alloyhaving a large reflectance increases the modulation degree leading to asuperior reproduction characteristic. A similar characteristic isexhibited by the Ag alloy. In this case, if the content of elementsother than the main components is in the range of not less than 5 atomic% but not more than 30 atomic % like the Al alloy, the rewritecharacteristic is improved further.

The preferable material of the second reflective layer is Al—Ti, Al—Ag,Al—Cu, Al—Cr or the like material containing an Al alloy as a maincomponent. Al can also be used.

From this, it has been found that when the content of elements otherthan Al in the Al alloy is in the range of not less than 0.5 atomic %but not more than to 4 atomic %, the characteristic of a multiplicity ofoverwrite cycles and the bit error rate are improved, and theimprovement is further enhanced in the case where the content is in therange of not less than one atomic % but not more than two atomic %. Asimilar characteristic is obtained for other Al alloys than describedabove.

Also, a layer may be used which is composed of the element unit such asAu, Ag, Cu, Ni, Fe, Co, Cr, Ti, Pd, Pt, W, Ta, Mo, Sb, Bi, Dy, Cd, Mn,Mg or V or an alloy containing any one of these elements as a maincomponent such as an Au alloy, Ag alloy, Cu alloy, Pd alloy or Pt alloyor an alloy between these alloys. In this way, the second reflectivelayer is composed of a metal element, a metalloid element, an alloy or amixture thereof.

Among these materials, those having a large heat conductivity such asCu, Al, Au, Cu alloy, Al alloy and Au alloy have a superior rewritecharacteristic as the disk can be cooled rapidly with ease. A similarcharacteristic is observed also for Ag and Ag alloy. In the case wherethe content of the elements other than Cu, Au and Ag making up the maincomponents, like the Al alloy, is in the range of not less than 0.5atomic % but not more than 4 atomic %, the characteristic of amultiplicity of overwrite cycles and the bit error rate are improved.This trend is further enhanced when the content is in the range of notless than one atomic % but not more than 2 atomic %.

Also, a study of the refractive index (n) and the extinction coefficient(k) of the materials of the first reflective layer and the secondreflective layer described above shows that the jitter increase after100 thousand overwrite cycles can be suppressed within 3% in the casewhere n of the first reflective layer is larger than n of the secondreflective layer and k of the first reflective layer is smaller than kof the second reflective layer.

The materials of the first reflective layer and the second reflectivelayer desirably represent at least 95% of the total number of atoms ofthe respective reflective layer. In the case where impurities other thanthe materials described above reach 5 atomic % or more, the possiblenumber of overwrite cycles is reduced to one half or otherwise therewrite characteristic is deteriorated.

The thickness of the first reflective layer is desirably not less than 5nm but not more than 100 nm. The thickness of the second reflectivelayer, on the other hand, is desirably not less than 30 nm but not morethan 200 nm.

Examples of desirable combinations of the materials of the firstreflective layer and the second reflective layer are an Al₉₄Cr₆ for thefirst reflective layer with Al₉₉Ti₁ for the second reflective layer,Al₉₀Ti₁₀ for the first reflective layer with Al₉₈Ti₂ for the secondreflective layer, Al₇₅Ti₂₅ for the first reflective layer with Al₉₉Ti₁for the second reflective layer, etc. in which case the first reflectivelayer and the second reflective layer contain the same main componentelement, and elements other than the main component element of Al arecontained more in the second reflective layer than in the firstreflective layer. A similar characteristic is obtained from thecombinations of Al—Ti with Al—Ti, Al—Cr with Al—Cr or other combinationssuch as Al—Ag with Al—Cu in which the Al alloy is a main component. TheAu alloy, Ag alloy, Cu alloy or a similar composition can improve therewrite characteristic of a multiplicity of overwrite cycles.

(21) The substrate material may be a polycarbonate substrate with atracking groove formed directly in the surface thereof, polyolefin,epoxy, acrylic resin, or a chemically reinforced glass having thesurface thereof formed with an ultraviolet setting resin layer.

The substrate having a tracking groove is the one with the whole or partof the substrate surface having a groove at least λ/10n′ (n′: refractiveindex of the substrate material) deep where λ is therecording/reproduction wavelength. The groove may be formed eithercontinuously over the whole periphery or segmented midway. It has beenfound that crosstalks are desirably reduced when the groove depth isabout λ/6n′. Further, it has been found that although the yield forsubstrate production is deteriorated but the cross erase is reduceddesirably when the groove is deeper than about λ/3n′.

Also, the groove may have different widths at different places. Asubstrate of sample servo format lacking a groove or of other trackingtypes or formats will do. A substrate having a format capable orrecording and reproduction in both grooves and lands or a substratehaving a format capable of recording and reproduction only in grooves orlands can also be used. The disk size is not limited to 12 cm, but othersizes including 13 cm, 3.5′, 2.5′, etc. are applicable with equaleffect. The disk thickness is neither limited to 0.6 mm but otherthickness such as 1.2 mm or 0.8 mm can be employed.

Two disk members including a first disk member and a second disk memberare fabricated by exactly the same method, and are attached to eachother by an adhesive with the second reflective layers thereof face toface. As an alternative, the second disk member may be replaced by adisk member of another configuration or a protective substrate. In thecase where the disk member used for attachment or the protectivesubstrate has a large transmittance in the ultraviolet wavelength area,the ultraviolet setting resin may be used for attaching the diskmembers. Other methods of attaching may also be used. A disk member of astructure having no second reflective layer may be attached with anadhesive layer formed on the topmost layer.

The first and second disk members described above are attached to eachother with the second reflective layers thereof face to face through theadhesive layer. The error rate is reduced further by coating theultraviolet setting resin about 10 μm thick on the second reflectivelayers of the first and second disk members beforehand and attaching thedisk members to each other after the resin is set.

Instead of attaching the first and second disk members to each other,the ultraviolet setting resin may be coated to the thickness of about 10μm on the second reflective layer of the first disk member. In the caseof a disk member of a structure lacking the second reflective layer, theultraviolet setting resin may be applied on the topmost layer.

(22) In addition to the structures described above, the structures ofthe disks 1 to 39 described below have a smaller remanence due to theabsorption control layer and have the effect of reducing the jitter.

Disk 1: Substrate 1, heat diffusion layer 2, protective layer 3, lowersurface protect layer 4, recording film 5, upper surface protect layer6, absorption control layer 7, first reflective layer 8, adhesive layer10

Disk 2: Substrate 1, heat diffusion layer 2, protective layer 3, lowersurface protect layer 4, recording film 5, upper surface protect layer6, absorption control layer 7, second reflective layer 9, adhesive layer10

Disk 3: Substrate 1, heat diffusion layer 2, protective layer 3, lowersurface protect layer 4, recording film 5, upper surface protect layer6, absorption control layer 7, adhesive layer 10

Disk 4: Substrate 1, heat diffusion layer 2, protective layer 3, lowersurface protect layer 4, recording film 5, absorption control layer 7,first reflective layer 8, second reflective layer 9, adhesive layer 10

Disk 5: Substrate 1, heat diffusion layer 2, protective layer 3, lowersurface protect layer 4, recording film 5, absorption control layer 7,first reflective layer 8, adhesive layer 10

Disk 6: Substrate 1, heat diffusion layer 2, protective layer 3, lowersurface protect layer 4, recording film 5, absorption control layer 7,second reflective layer 9, adhesive layer 10

Disk 7: Substrate 1, heat diffusion layer 2, protective layer 3, lowersurface protect layer 4, recording film 5, absorption control layer 7,adhesive layer 10

Disk 8: Substrate 1, heat diffusion layer 2, protective layer 3,recording film 5, upper surface protect layer 6, absorption controllayer 7, first reflective layer 8, second reflective layer 9, adhesivelayer 10

Disk 9: Substrate 1, heat diffusion layer 2, protective layer 3,recording film 5, upper surface protect layer 6, absorption controllayer 7, first reflective layer 8, adhesive layer 10

Disk 10: Substrate 1, heat diffusion layer 2, protective layer 3,recording film 5, upper surface protect layer 6, absorption controllayer 7, second reflective layer 9, adhesive layer 10

Disk 11: Substrate 1, heat diffusion layer 2, protective layer 3,recording film 5, upper surface protect layer 6, absorption controllayer 7, adhesive layer 10

Disk 12: Substrate 1, heat diffusion layer 2, protective layer 3,recording film 5, absorption control layer 7, first reflective layer 8,second reflective layer 9, adhesive layer 10

Disk 13: Substrate 1, heat diffusion layer 2, protective layer 3,recording film 5, absorption control layer 7, first reflective layer 8,adhesive layer 10

Disk 14: Substrate 1, heat diffusion layer 2, protective layer 3,recording film 5, absorption control layer 7, first reflective layer 8,second reflective layer 9, adhesive layer 10

Disk 15: Substrate 1, heat diffusion layer 2, protective layer 3,recording film 5, absorption control layer 7, adhesive layer 10

Disk 16: Substrate 1, protective layer 3, lower surface protect layer 4,recording film 5, upper surface protect layer 6, absorption controllayer 7, first reflective layer 8, second reflective layer 9, adhesivelayer 10

Disk 17: Substrate 1, protective layer 3, lower surface protect layer 4,recording film 5, upper surface protect layer 6, absorption controllayer 7, first reflective layer 8, adhesive layer 10

Disk 18: Substrate 1, protective layer 3, lower surface protect layer 4,recording film 5, upper surface protect layer 6, absorption controllayer 7, second reflective layer 9, adhesive layer 10

Disk 19: Substrate 1, protective layer 3, lower surface protect layer 4,recording film 5, upper surface protect layer 6, absorption controllayer 7, adhesive layer 10

Disk 20: Substrate 1, heat diffusion layer 2, lower surface protectlayer 4, recording film 5, upper surface protect layer 6, absorptioncontrol layer 7, first reflective layer 8, second reflective layer 9,adhesive layer 10

Disk 21: Substrate 1, heat diffusion layer 2, lower surface protectlayer 4, recording film 5, upper surface protect layer 6, absorptioncontrol layer 7, first reflective layer 8, adhesive layer 10

Disk 22: Substrate 1, heat diffusion layer 2, lower surface protectlayer 4, recording film 5, upper surface protect layer 6, absorptioncontrol layer 7, second reflective layer 9, adhesive layer 10

Disk 23: Substrate 1, heat diffusion layer 2, lower surface protectlayer 4, recording film 5, upper surface protect layer 6, absorptioncontrol layer 7, adhesive layer 10

Disk 24: Substrate 1, protective layer 3, lower surface protect layer 4,recording film 5, upper surface protect layer 6, absorption controllayer 7, first reflective layer 8, second reflective layer 9, adhesivelayer 10

Disk 25: Substrate 1, protective layer 3, lower surface protect layer 4,recording film 5, upper surface protect layer 6, absorption controllayer 7, first reflective layer 8, adhesive layer 10

Disk 26: Substrate 1, protective layer 3, lower surface protect layer 4,recording film 5, upper surface protect layer 6, absorption controllayer 7, second reflective layer 9, adhesive layer 10

Disk 27: Substrate 1, protective layer 3, lower surface protect layer 4,recording film 5, upper surface protect layer 6, absorption controllayer 7, adhesive layer 10

Disk 28: Substrate 1, protective layer 3, lower surface protect layer 4,recording film 5, absorption control layer 7, first reflective layer 8,second reflective layer 9, adhesive layer 10

Disk 29: Substrate 1, protective layer 3, lower surface protect layer 4,recording film 5, absorption control layer 7, first reflective layer 8,second reflective layer 9, adhesive layer 10

Disk 30: Substrate 1, protective layer 3, lower surface protect layer 4,recording film 5, absorption control layer 7, first reflective layer 8,adhesive layer 10

Disk 31: Substrate 1, protective layer 3, lower surface protect layer 4,recording film 5, absorption control layer 7, adhesive layer 10

Disk 32: Substrate 1, protective layer 3, recording film 5, uppersurface protect layer 6, absorption control layer 7, first reflectivelayer 8, second reflective layer 9, adhesive layer 10

Disk 33: Substrate 1, protective layer 3, recording film 5, uppersurface protect layer 6, absorption control layer 7, second reflectivelayer 9, adhesive layer 10

Disk 34: Substrate 1, protective layer 3, recording film 5, uppersurface protect layer 6, absorption control layer 7, first reflectivelayer 8, adhesive layer 10

Disk 35: Substrate 1, protective layer 3, recording film 5, uppersurface protect layer 6, absorption control layer 7, adhesive layer 10

Disk 36: Substrate 1, protective layer 3, recording film 5, uppersurface protect layer 6, absorption control layer 7, first reflectivelayer 8, second reflective layer 9, adhesive layer 10

Disk 37: Substrate 1, protective layer 3, recording film 5, absorptioncontrol layer 7, second reflective layer 9, adhesive layer 10

Disk 38: Substrate 1, protective layer 3, recording film 5, absorptioncontrol layer 7, first reflective layer 8, adhesive layer 10

Disk 39: Substrate 1, protective layer 3, recording film 5, absorptioncontrol layer 7, adhesive layer 10

(23) The recording/reproduction characteristic is improved simply bysecuring a desired range of thickness or material of each layerindependently. A higher effect can be achieved, however, by combiningthe desired ranges of the respective factors.

(24) A better characteristic is obtained in the case where the recordingfilm has a composition defined as 0.12≦x≦0.24, 0.20≦y≦0.31, 0.54≦z≦0.58, 0≦w≦0.04.

Further, in the case where the Ge amount reaches not less than 20 atomic% in this range, the read light endurance is improved by 1.5 times. Theread light endurance is obtained by determining, and by comparison with,the power of the read light for reducing the recording signal by 2 dB ormore during a five-minute reproduction. For the Ge amount of not morethan 17 atomic %, on the other hand, the extinction ratio is large alsoin the case where the linear speed is high, thus producing a superiorfigure of not less than 30 dB for 12 m/s.

In the case where M is Ag, the recording sensitivity is improved by 10%as compared with Ge—Sb—Te. In the case where M is at least one of Cr, Wand Mo, on the other hand, the possible number of overwrite cycles atwhich the jitter increases at least 5% is improved three times or morein a multiplicity of overwrite cycles, as compared with Ge—Sb—Te. In thecase where M is at least one of Pt, Co and Pd, the crystallizationtemperature is increased by at least 50° C. as compared with Ge—Sb—Te.

Also, in the case where the impurities elements in the recording filmare not more than 5 atomic %, the deterioration of the rewritecharacteristic can be reduced desirably. The figure of not more than 2atomic % produces a more desirable result.

The thickness of the recording film is desirably not less than 10 nm butnot more than 30 nm, and the figure of not less than 13 nm but not morethan 20 nm is more desirable.

Though somewhat time-consuming, mixing nitrogen with the sputtering gasat the start or end of the fabrication process of the recording film orusing a target mixed with a small amount of nitrogen in the compositionof the recording film or otherwise containing nitrogen in theneighborhood of the boundary between the recording film and other layersimproves the adhesion for an improved characteristic.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a sectional view showing a structure of an informationrecording medium according to embodiment 1 of the present invention.

FIG. 2 is a sectional view showing an information recording mediumhaving the current structure.

FIG. 3 shows recording waveforms used for evaluation of therecording/reproduction characteristic of an information recording mediumaccording to the present invention.

FIG. 4 shows the rewrite characteristic of an information recordingmedium according to the invention and an information recording medium ofthe current structure.

FIG. 5 is a sectional view showing a structure of an informationrecording medium according to embodiment 3 of the invention.

FIG. 6 is a sectional view showing a structure of an informationrecording medium according to embodiment 5 of the invention.

FIG. 7 is a sectional view showing a structure of an informationrecording medium according to embodiment 6 of the invention.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention will be explained in detail below with referenceto embodiments.

The reference numerals used in the drawings are defined below.

1, 1′: Substrate 2, 2′: Heat diffusion layer 3, 3′: Protective layer 4,4′: Lower surface protect layer 5, 5′: Recording film 6, 6′: Uppersurface protect layer 7, 7′: Absorption control layer 8, 8′: Firstreflective layer 9, 9′: Second reflective layer 10: Adhesive layer 11,11′: Reflective layer T: Window width (Tw) Pr: Low power level Pe:Intermediate power level Ph: High power level Pp: Preheat power levelP1: Level of power 0 Tc: Cooling pulse width Tp: Preheat level width

Other reference numerals are defined in the drawings.

(1) Embodiment 1

(Configuration and Fabrication Method)

FIG. 1 is a sectional view showing a structure of a disk-typeinformation recording medium according to embodiment 1 of thisinvention. This medium was fabricated in the following manner.

First, a heat diffusion layer 2 of Al₂O₃ film was formed in thethickness of about 30 nm on a polycarbonate substrate 12 cm in diameterand 0.6 mm thick having a tracking groove in the surface thereof. Then,a protective layer 3 of ZnS film about 45 nm thick, a lower surfaceprotect layer 4 of SiO₂ film about 5 nm thick, a recording film 5 ofGe₁₄Sb₂₉Te₅₇ about 15 nm thick, an upper surface protect layer 6 of SiO₂film about 15 nm thick, an absorption control layer 7 of MO₈₀ (SiO₂)₂₀film about 18 nm thick, a first reflective layer 8 of Al₈₉Ti₁₁ about 20nm thick, and a second reflective layer 9 of Al₉₈Ti₂ film about 180 nmthick were formed sequentially. The film lamination was formed by amagnetron sputtering apparatus. In this way, a first disk member wasobtained.

On the other hand, a second disk member having the same configuration asthe first disk member was obtained by exactly the same method. Thesecond disk member was produced in such a manner that a heat diffusionlayer 2′ of Al₂O₃ film, a protective layer 3′ of ZnS film about 45 nmthick, a lower surface protect layer 4′ of SiO₂ film about 5 nm thick, arecording film 5′ of Ge₁₄Sb₂₉Te₅₇ about 15 nm thick, an upper surfaceprotect layer 6′ of SiO₂ film about 15 nm thick, an absorption controllayer 7′ of Mo₈₀(SiO₂)₂₀ film about 18 nm thick, a first reflectivelayer 8′ of Al₈₉Ti₁₁ film about 20 nm thick, and a second reflectivelayer 9′ of a Al₉₈Ti₂ film about 180 nm thick, were sequentially formedon a substrate 1′ having a diameter of 12 cm and 0.6 mm thick.

After that, the first disk member and the second disk member wereattached to each other with the second reflective layers 9, 9′ thereofface to face through an adhesive layer 10, thereby producing a disk-typeinformation recording medium shown in FIG. 1.

(Initial Crystallization)

The recording films 5, 5′ of the medium fabricated in the mannerdescribed above were subjected to initial crystallization in thefollowing manner. Only the recording film 5 which is treated exactly thesame way as the recording film 5′ is described below.

The medium is rotated so that the linear speed of a point on therecording track is 8 m/s. The laser light power of a semiconductor laser(about 810 nm in wavelength) with a laser light power of 800 mW havingan elliptical spot long radially of the medium was radiated on therecording film 5 through the substrate 1. The spot was moved by beingdisplaced by ¼ of the spot length each time radially of the medium. Bydoing so, the initial crystallization was effected. One session of theinitial crystallization is sufficient. Nevertheless, three repeatedsessions of the initial crystallization could somewhat reduce the noiseincrease due to the initial crystallization. This initialcrystallization can be advantageously carried out at high speed.

(Recording, Erasure and Reproduction)

Then, while conducting the tracking and automatic focusing operation inthe recording area of the recording film 5 for which the initialcrystallization was completed in the manner described above, informationwas recorded by changing the power of the recording laser light betweenthe intermediate power level (4.5 mW) and the high power level Ph (11mW). The linear speed of the recording track is 9 m/s, thesemi-conductor laser wavelength is 636 nm, and the numerical aperture(NA) of the lens is 0.6. The amorphous or similar portion formed in therecording area by the recording laser light constitutes a recordingpoint. The reflectance of this medium is higher in crystal state, andthe area which turned amorphous by recording has a lower reflectance.

The power ratio in the range of 1:0.3 to 1:0.6 between high level andintermediate level of the recording laser light is especiallypreferable. Also, other power levels can be employed for each shorttime. The recording/reproduction are performed by a device having meansin which, as shown in FIG. 3, while one recording mark is being formed,the power is repeatedly reduced to a level lower than the intermediatepower level by one half of the window width (Tw/2) each time, and awaveform having a preheat level Pp (4.6 mW) at the head of the recodringpulse is generated. Then, a reproduced signal waveform was obtainedwhich has an especially low jitter value and low error rate. The preheatlevel is slightly higher than the intermediate level and lower than thehigh level. This waveform has the feature that the preheat level widthTp (length for which the preheat level is held) changes by thecombination of the recording mark and the length of the space formedimmediately before the recording mark and the feature that the coolingpulse width Tc (time width during which the level is reduced to Pr atthe end of the recording pulse) is determined by the combination of therecording mark and the length of the space following the particularmark. The shorter the space immediately before the mark and the longerthe mark, the longer the Tp, while the longer the space immediatelybefore the mark and the shorter the mark, the longer the width Tp.Depending on the structure of the medium, however, in the case where Tpof the recording waveform of the 6 Tw mark is especially long, thejitter can be reduced effectively. Also, the longer the space followingthe mark and the longer the mark, the shorter the width Tc, while theshorter the space following the mark and the shorter the mark, the longthe Tc.

In FIG. 3, only the recording waveforms of 3 Tw, 4 Tw, 6 Tw and 11 Tware shown. The waveform of 5 Tw is such that among a series of the pulsetrain having a high power level in the recording waveform of 6 Tw, oneeach of the high power level Ph of Tw/2 and the immediately followinglow power level Ph of Tw/2 are removed. In the recording waveforms 7 Twto 10 Tw, on the other hand, a set of the high power level Ph of Tw/2and the low power level Ph of Tw/2 is added immediately before the pulseof high power level at the tail end of the recording waveform of 6 Tw.It follows, therefore, that the recording waveform of 11 Tw is theresult of adding five such sets. The length of the shortest recordingmark corresponding to 3 Tw is set to 0.42 μm. Once the portion to berecorded is passed, the laser light power is reduced to the low powerlevel Pr (1.5 mW) for reproduction (reading). The recording signalcontains dummy data with a repetition of the 4T mark and the 4T space atthe starting end and the tail end of the information signal. Thestarting end also contains VFO.

In this recording method, the portion where information is alreadyrecorded can be rewritten into new information by overwriting the newinformation without erasure. In other words, the overwrite operationwith a substantially circular single light spot is possible.

As an alternative, in the first one or a plurality of disk rotationsduring the overwrite operation, the continuous light of the intermediatepower level (4.5 mW) or a power level proximate thereto of thepower-modulated recording laser light is radiated thereby to erase therecorded information, and then during the next one rotation, therecording operation is performed by radiating the laser lightpower-modulated in accordance with the information signal, between thelow power level (1.5 mW) and the high power level (11 mW) or between theintermediate power level (4.5 mW) and the high power level (11 mW). Byrecording after erasing the information in this way, the remanence ofthe information previously written is reduced. Thus, the overwriteoperation becomes easy even when the linear speed is increased to twice.

These methods are effective not only with the recording film used forthe medium according to the invention but also with the recording filmof other media.

When the recording and erase operations are repeated in the informationrecording medium according to this embodiment, the jitter (σ/Tw) isreduced by 6% or more as compared with the conventional informationrecording medium described in embodiment 2 for each overwrite cycle, asshown in FIG. 4. The jitter is an indicator of the degree to which thereproduction signal fluctuates with respect to the window width (Tw)when reproducing the position of the edge portion of the recording mark.With the increase in the jitter value, the detection position of theedge portion represents a major proportion of the window width andtherefore the recording signal cannot be accurately reproduced. Thus,the jitter is desirably as small as possible. The reason why the jitterhas been reduced is that the absorption can be controlled by theabsorption control layer so that the remanence can be reduced also atthe time of recording with high linear speed.

Observing the recording mark under the transmission electron microscope,the mark size (mark area) was compared between the case in which thelong mark (amorphous state) is rewritten and the case in which the longspace (crystal state) is overwritten. In the case of the informationrecording medium according to this embodiment, the former has been foundto be substantially the same as the latter. Under the strong absorptioncontrol, however, the former is slightly smaller than the latter. Forthe information recording medium described in embodiment 2, on the otherhand, the former is larger than the latter.

When recording information on an information recording medium, thenumber of times of recording at one point of an information recordingmedium is generally said to be about 100 thousands. In this embodiment,therefore, the recording/reproduction characteristic from the firstrecording to the end of 100 thousands of overwrite cycles was studied.

The window width (Tw) in jitter measurement is 16 ns, the shortestrecording signal is 3 Tw and the longest recording signal is 11 Tw,which are randomly recorded. A reproduction equalizing circuit was usedfor the measurement.

The absorption control layer, which is effective also in other recordingschemes, has an especially high effect of reducing the jitter byrecording the edge portion accurately in the mark edge recording. Themark edge recording is a recording scheme in which the edge portion of arecording mark is regarded as “1” while the space between marks and theinterior of the marks are regarded as “0”. Further, the effect isconspicuous where the linear speed is higher than 6 m/s.

The effect of the absorption control layer, though observed also at thetime of low density recording, becomes conspicuous at the time of highdensity recording. An example is the case of recording on a land-grooverecording substrate at a track pitch of not less than 0.53 μm but notmore than 0.65 μm and/or with the shortest mark length of not less than0.39 μm but not more than 0.45 μm. The modulation degree is increasedand the rewrite characteristic is desirably improved for therecording/reproduction wavelength of not less than 600 nm but not morethan 660 nm. Also for the wavelength shorter than 600 nm, the mediumaccording to this embodiment is usable by correcting the film thicknessin accordance with the wavelength ratio.

(Absorption Control Layer)

In this embodiment, changing the thickness of the film used in theabsorption control layers 7, 7′, the jitter (σ/Tw) after 10 overwritecycles was measured, and the following result was obtained.

The square mean value (%) of the jitter and the modulation degree (%) ofthe front edge and the trailing edge after ten overwrite cycles areindicated with respect to the thickness (nm) of the absorption controllayer. Unless otherwise specified, the jitter is expressed by the squaremean of the jitter values at the front edge and the trailing edge.

The modulation degree (Mod) was calculated according to the followingequation.

Mod(%)=100×(Ic−Ia)/Ic

where Ic is the highest level of reflectance of the crystal (erased)state at the time of EFM signal recording and Ia the lowest level ofreflectance of the amorphous (recorded) state at the time of EFM signalrecording.

Mod(%)=100×(Ic−Ia)/Ic

Absorption control Jitter after 10 Modulation layer thickness (nm)overwrite cycles (%) degree (%)  2 25 —  5 20 — 10 15 53 20 13 51 40 1347 50 — 43 60 — 40

This indicates that in the case where the thickness of the absorptioncontrol layer is reduced, the jitter after 10 overwrite cycles increaseswhile in the case where the thickness of the absorption control layer isincreased, the modulation degree is increased. The jitter is increasedwith the decrease in the absorption control layer thickness, probablybecause the reduction in the absorption ratio (Ac/Aa), i.e. the ratiobetween the absorption coefficient Ac of the recording film in crystalstate and the absorption coefficient Aa of the recording film inamorphous state fails to sufficiently control the absorption, and theremanence is caused. The absorption ratio (Ac/Aa), which cannot beactually measured, was determined by optical calculations. The resultshows that the thickness of the absorption control layer is desirablynot less than 5 nm but not more than 50 nm, and more desirably not lessthan 10 nm but not more than 40 nm.

In this embodiment, optical calculations were carried out by changingthe input values n and k and the optical constants of the film used forthe absorption control layers 7, 7′. First, maintaining k at 1.7 whilechanging n, the absorption ratio (Ac/Aa) was determined as follows.

n Ac/Aa 0.5 0.9 1.2 1.0  1.8 1.05 2.0 1.11 3.0 1.13 4.5 1.12 5.5 1.076.0 1.00 6.5 0.9 

This indicates that with the change in n of the absorption controllayer, the absorption ratio (Ac/Aa) changes. Therefore, n of theabsorption control layer is desirably not less than 1.2 but not morethan 6 and more desirably not less than 1.8 but not more than 5.5.

Then, maintaining n at 3.3 and changing k, the absorption ratio (Ac/Aa)was determined as follows.

k Ac/Aa 0   0.97 0.5 1.02 0.8 1.1  1.5 1.11 1.8 1.13 2.5 1.13 3.0 1.083.3 1.01 4.5 0.95

This indicates that with the change in k of the absorption controllayer, the absorption ratio (Ac/Aa) changes. Therefore, k of theabsorption control layer is desirably not less than 0.5 but not morethan 3.3 and more desirably not less than 0.8 but not more than 3.

According to this embodiment, the jitter (σ/Tw) and the recordingsensitivity after 10 overwrite cycles were measured by changing thecomposition ratio of Mo—(SiO₂) used for the absorption control layers 7,7′, and the following result was obtained. The recording sensitivity,which was based on Mo₈₀(SiO₂)₂₀, is indicated as + when improved, as −when deteriorated and as 0 when unchanged.

Absorption control Recording layer composition Jitter (%) sensitivity(%) Mo₂₀(SiO₂)₈₀ 25 Not measured Mo₃₅(SiO₂)₆₅ 22 Not measuredMo₄₂(SiO₂)₅₈ 20 +10 Mo₅₀(SiO₂)₅₀ 18 +10 Mo₆₁(SiO₂)₃₉ 15 +5 Mo₆₇(SiO₂)₃₃14 +3 Mo₇₂(SiO₂)₂₈ 13 0 Mo₈₀(SiO₂)₂₀ 13 0 Mo₉₀(SiO₂)₁₀ — 0 Mo — −5

This indicates that with the increase in the amount of Mo with respectto the composition of the absorption control layer, the jitter after 10overwrite cycles can be reduced. The jitter is reduced probably byreason of the fact that the absorption ratio (Ac/Aa) is so large thatthe remanence is hard to occur. The Mo amount that represents of thetotal composition of the absorption control layer, therefore, ispreferably not less than 42 mol %. Also, Mo itself has a larger heatconductivity than Mo—(SiO₂) and therefore, the use of Mo alone, somewhatreduces the recording sensitivity. The desirable amount of Mo is notless than 61 mol % but not more than 90 mol %.

In this embodiment, the Mo—(SiO₂) film used for the absorption controllayers 7, 7′ was analyzed by the X-ray photoelectron spectrography, andit was found that the Mo—(SiO₂) film is composed mainly of a metal (Mo)and a dielectric material (SiO₂). Depending on the composition ratio,the peak of the metal Mo sightly lowers with the peak corresponding tothe Mo oxide appearing. However, the main peak appears at the positionof the metal Mo, and if an oxide, oxygen is not saturated. This isbecause oxygen is exchanged between the metal (Mo) and the dielectricmaterial (SiO₂) in the absorption control layer, and the compositionratio for the compound as a whole is Mo—(SiO₂). In the case where thedielectric material is a compound other than the oxide, the same effectis obtained as if oxygen has directly replaced other elements. A sulfidesuch as Sb₂S₃, for example, includes Mo and Sb₂S₃, and depending on thecomposition ratio, a peak corresponding to the Mo sulfide appears otherthan the peak of the metal Mo.

As described above, it was found that the absorption control layer iscomposed of a metal element or an unsaturated metal oxide and adielectric material.

A similar result was obtained by use of Cr, W, Fe, Sb, Mn, Ti, Co, Ge,Pt, Ni, Nb, Pd, Be or Ta as a replacement material of Mo in theMo—(SiO₂) film used for the absorption control layers 7, 7′. Among theseelements, Re and W are high in melting point and more desirable. Pd andPt, on the other hand, are less reactive with other layers and thereforehave desirably a further increased possible number of overwrite cycles.The use of Ni or Co makes an inexpensive target usable as compared withother elements, and can reduce the total production cost. Cr and Ti havea high anticorrosiveness and the result of the life test on them wassuperior to that of others. Also, Tb, Gd, Sm, Cu, Au, Ag, Ca, Al, Zr,Ir, Hf, etc. are also usable.

In this embodiment, the materials usable instead of SiO₂ in theMo—(SiO₂) film used with the absorption control layers 7, 7′ are oxidesincluding SiO, Al₂O₃, BeO, Bi₂O₃, CoO, CaO, Cr₂O₃, CeO₂, Cu₂O, CuO, CdO,Dy₂O₃, FeO, Fe₂O₃, Fe₃O₄, GeO, GeO₂, HfO₂, In₂O₃, La₂O₃, MgO, MnO, MoO₂,MoO₃, NbO, NbO₂, NiO, PbO, PdO, SnO, SnO₂, Sc₂O₃, SrO, ThO₂, TiO₂,Ti₂O₃, TiO, Ta₂O₅, TeO₂, VO, V₂O₃, VO₂, WO₂, WO₃, Y₂O₃ and ZrO₂,nitrides including AlN, Bn, CrN, Cr₂N, GeN, HfN, Si₃N₄, Al—Si—N groupmaterial (such as AlSiN₂), Si—N group material, Si—O—N group material,TaN, TiN and ZrN, sulfides including ZnS, Sb₂S₃, CdS, In₂S₃, Ga₂S₃, GeS,SnS₂, PbS, Bi₂S₃, SrS, MgS, CrS, CeS and TaS, selenides including SnSe₂,Sb₂Se₃, CdSe, ZnSe, In₂Se₃, Ga₂Se₃, GeSe, GeSe₂, SnSe, PbSe and Bi₂Se₃,fluorides including CeF₃, MgF₂, CaF₂, TiF₃, NiF₃, FeF₂ and FeF₃, Si, Ge,borides including TiB₂, B₄C, B, CrB, HfB₂, TiB₂ and WB, carbidesincluding C, Cr₃C₂, Cr₂₃C₆, Cr₇C₃, Fe₃C, Mo₂C, WC, W₂C, HfC, TaC andCaC₂, or a material having a composition similar to the materialsdescribed above or a mixture thereof. In addition, In—Sb, Ga—As, In—P,Ga—Sb, In—As, etc. could also be used.

Among these materials, the use of SiO₂, Ta₂O₃, Y₂O₃—ZrO₃ or the likeoxides makes it possible to use a target less expensive than when usingother materials, and therefore can reduce the total cost of production.Among the oxides, SiO₂, Ta₂O₅, Y₂O₃—ZrO₂ are less reactive and desirablyhave an increased number of overwrite cycles. BeO and Cr₂O₃ are alsodesirable as they have a high melting point. Al₂O₃ is high in heatconductivity, and therefore, in the case where it is made into a diskhaving a structure lacking the first reflective layer and/or the secondreflective layer, is less deteriorated than other materials in therewrite characteristic.

The use of a nitride, on the other hand, increases the adhesion with thelayers adjoining the absorption control layer and becomes resistant toan external shock. When a sulfide or selenide is used, on the otherhand, the sputter rate can be increased for a shortened film-makingtime. In the case where a carbide is used, the hardness of theabsorption control layer is increased thereby to suppress the flow ofthe recording film for a multiplicity of overwrite cycles.

A metal element and/or a dielectric material, if having a melting pointhigher than the melting point (about 600° C.) of the recording film, cansuppress the jitter increase at the time of ten thousand overwritecycles. In the case where the two materials have a melting point of notlower than 600° C., the jitter increase can be desirably suppressed tonot more than 3%.

Also, in the case where the impurities elements in the absorptioncontrol layer exceeds 2 atomic % of the components of thereof, thejitter at the front or trailing edge after 10 overwrite cycles is foundto exceed 15%. Further, when the impurities elements exceed 5 atomic %,the jitter is found to increase to 18% or more. Thus, the deteriorationof the rewrite characteristic can be desirably reduced when theimpurities elements contained in the absorption control layer is notmore than 5 atomic %. The figure of not more than 2 atomic % is moredesirable.

(Measurement of Optical Constants of Absorption Control Layer)

Separating the disk member between the upper surface protect layer andthe absorption control layer, the reflectance was studied for therecording/reproduction wavelength. Specifically, the second reflectivelayer 9 about 180 nm thick, the first reflective layer 8 of Al₈₉Ti₁₁film about 20 nm thick and the absorption control layer 7′ of MO₈₀(SiO₂)₂₀ film about 18 nm thick are deposited on the adhesive layer 10.

Then, by the counter sputtering (Ar gas etching) with the sputteringdevice, the reflectance was measured with the MO₈₀(SiO₂)₂₀ film reducedin thickness. The thickness of the film etched was masked partly at thetime of etching, and after etching, the mask was removed and thereflectance was measured with step meter. After repeating this operationtwice, the following values of the absorption control layer thicknessand the reflectance were obtained.

Absorption control layer thickness Reflectance (%) 18 27 10 49  0 75

As to n and k of the reflective layer, on the other hand, the surface ofthe reflective layer was exposed by separation and the values n and kwere determined by the ellipsometry with variable wavelengths.

From the reflectance and n and k of the reflective layer thus obtained,the values n and k which indicate the reflectance of structures havingdifferent thicknesses of the absorption control layer were determined bycalculation. As a result, it has been found that n is 3.3 and k is 1.3.

(Surface Protect Layers)

A desirable material replacing SiO₂ of the upper surface protect layer 6and the lower surface protect layer 4 is Al₂O₃ or a mixture of Al₂O₃ andSiO₂. In the case where the SiO₂ or Al₂O₃ content is not less than 70mol %, the crystallization rate is increased, so that the extinctionratio increases to not less than 25 dB even for the speed of 18 m/swhich is about twice as high as the speed in the absence of the surfaceprotect layer.

The next desirable materials replacing SiO₂ of the upper surface protectlayer 6 and the lower surface protect layer 4 are Ta₂O₅, a mixturebetween Ta₂O₅ and SiO₂ or Al₂O₃, followed by ZrO₂—Y₂O₃, SiO₂ or amixture thereof with Al₂O₃ or Ta₂O₅. Among them, Al₂O₃ can suppress thefluctuations of the reflectance level during a multiplicity of overwritecycles to not more than 5%, thus reducing the jitter desirably. CoO,Cr₂O₃ and NiO are more desirable as a uniform crystal grain size isobtained by initial crystallization so that the jitter increases tolesser degree in the initial stage of overwrite operation.

Also, nitrides such as AlN, BN, CrN, Cr₂N, GeN, HfN, Si₃N₄, Al—Si—Ngroup material (such as AlSiN₂), Si—N group material, Si—O—N groupmaterial, TaN, TiN and ZrN increases the adhesion, and the informationrecording medium is desirably less deteriorated under external shocks. Amaterial having the same composition as the recording film containingnitrogen or a material of a similar composition also improves theadhesion.

Other materials usable include oxides including BeO, Bi₂O₃, CeO₂, Cu₂O,CuO, CdO, Dy₂O₃, FeO, Fe₂O₃, Fe₃O₄, GeO, GeO₂, HfO₂, In₂O₃, La₂O₃, MgO,MnO, MoO₂, MoO₃, NbO, NbO₂, PbO, PdO, SnO, SnO₂, Sc₂O₃, SrO, ThO₂, TiO₂,Ti₂O₃, TiO, TeO₂, VO, V₂O₃, VO₂, WO₂ and WO₃, and carbides including C,Cr₃C₂, Cr₂₃C₆, Cr₇C₃, Fe₃C, Mo₂C, WC, W₂C, HfC, TaC and CaC₂, or amaterial having a composition similar to the materials described above.

Also, a mixture of these materials may be used.

The upper surface protect layer 6, the lower surface protect layer 6, orthe materials used in place of the upper surface protect layer 6 and thelower surface protect layer 4 desirably represent not less than 90% ofall the atoms of the respective surface protect layers. In the casewhere the impurities other than these materials increases to 10 atomic %or more, the possible number of overwrite cycles is reduced by 50% ormore or otherwise the write characteristic is deteriorated.

In the absence of the upper surface protect layer, diffusion occurs intothe recording film of the reflective layer material, and the remanenceincreases, so that the reduction in the reflectance level due to 100thousand overwrite cycles is small and can be suppressed to 5% or less.With the change in the reflectance level, the reproduction signal leveldevelops an offset, and the jitter increases by the amount equal to theoffset resulting in an increased jitter. For this reason, the variationsof the reflectance level are better smaller.

Further, in order to maintain the modulation degree at 43% or more, thefigure of no more than 12 nm is desirable. For the figure of not morethan 5 nm, the modulation degree can be increased to 47% or more. Auniform film is formed with the figure of about 2 nm or more, andtherefore, when the thickness of the upper surface protect layer ismaintained between 2 and 12 nm, the recording/reproductioncharacteristic is desirably improved.

In the absence of the lower surface protect layer, diffusion occurs intothe recording film of a protective layer material, and the remanenceincreases, so that the jitter increases by more than 6% at the time of100 thousand overwrite cycles. Further, in order to maintain themodulation degree at 43% or more, the thickness is desirably not morethan 25 nm. For the thickness of 5 nm or more but not more than 10 nm,the modulation degree can be maintained at 47% or more. A uniform filmis formed when the thickness is not less than about 2 nm, and therefore,by maintaining the thickness of the lower surface protect layer between2 and 25 nm, the recording/reproduction characteristic can be improveddesirably.

(Protective Layer)

According to this embodiment, the protective layer 2 is formed of ZnS.

The materials that can replace ZnS of the protective layer 2 include aSi—N group material, a Si—O—N group material, oxides such as SiO₂, SiO,TiO₂, Al₂O₃, Y₂O₃, CeO₂, La₂O₃, In₂O₃, GeO, GeO₂, PbO, SnO, SnO₂, BeO,Bi₂O₃, TeO₂, WO₂, WO₃, Sc₂O₃, Ta₂O₅, ZrO₂, Cu₂O and MgO, nitrides suchas TaN, AlN, BN, Si₃N₄, GeN and Al—Si—N group material (such as AlSiN₂),sulfides such as ZnS, Sb₂S₃, CdS, In₂S₃, Ga₂S₃, GeS, SnS₂, PbS andBi₂S₃, selenides such as SnSe₂, Sb₂Se₃, CdSe, ZnSe, In₂Se₃, Ga₂Se₃,GeSe, GeSe₂, SnSe, PbSe and Bi₂Se₃, fluorides such as CeF₃, MgF₂ andCaF₂, or Si, Ge, TiB₂, B₄C, B, C, or materials having a similarcomposition to the materials described above. Also, ZnS—SiO₂, ZnS—Al₂O₃,etc. or a mixture layer or a multi-layer of these materials may be used.Among them, ZnS has a large n and can maintain a large modulationdegree. In the case of a mixture containing 60 mol % or more of thismaterial, the advantageous points of the large n of ZnS and the chemicalstability of the oxide are combined with each other. Further, ZnS has alarge sputter rate, so that when ZnS represents at least 80 mol %, thefilm-fabrication time can be shortened. Other sulfides and selenides canproduce a similar characteristic.

The desirable element ratio of these compounds, as expressed by theratio between metal element and oxygen for oxides and the ratio betweenmetal element and sulfide element for sulfides, is desirablyapproximately 2 to 3 for Al₂O₃, Y₂O₃ and La₂O₃, 1 to 2 for SiO₂, ZrO₂and GeO₂, 2 to 5 for Ta₂O₅ and 1 to 1 for ZnS. Nevertheless, a similareffect can be produced even when the ratio deviates from these figures.The deviation of the ratio from the integral ratio described above, ifany, is desirably not more than ±10 atomic % in Al amount from Al₂O₃ interms of the Al-to-O ratio in Al—O, not more than ±10 atomic % in Siamount from SiO₂ in terms of the Si-to-O ratio in Si—O. In this way, thedeviation of the metal element is desirably not more than 10 atomic %.Once the deviation increases to 10 atomic % or more, the resultingchange in the optical characteristics reduces the modulation degree by10% or more.

The material of the protective layer 2 and the replacement material ofthe protective layer 2 desirably represent at least 90% of all the atomsof each protective layer. In the case where the impurities other thanthese materials reach 10 atomic % or more, the possible number ofoverwrite cycles is reduced to one half or otherwise the rewritecharacteristic is deteriorated.

By changing the thickness of the protective layer used in thisembodiment, the modulation degree and the jitter (σ/Tw) after tenoverwrite cycles were measured, and the following result was obtained.The calculation formula for the modulation degree (Mod) is as follows.

Mod(%)=100×(Ic−Ia)/Ic

where Ic is the reflectance level of crystal (erased) state at the timeof EFM signal recording, and Ia is the reflectance level of theamorphous (recorded) state at the time of EFM signal recording.

Protective layer Modulation thickness (nm) degree (%) Jitter (%) 15 41 —20 43 — 35 48 15 45 51 14 60 50 15 70 — 18 80 — 22

The thickness of the protective layer is desirably 20 to 70 nm at whichthe modulation degree for recording can be increased to 43% or more. Thethickness of 35 to 60 nm is more desirable.

(Heat Diffusion Layer)

According to this embodiment, the heat diffusion layer 1 is formed ofAl₂O₃.

The desirable materials replacing Al₂O₃ of the heat diffusion layer 1are MgO, BeO, SiC, BN are B₄C which have a large heat conductivity.Also, Ta₂O₅, SiO₂, Al₂O₃ and a mixture of any combinations thereof havean inexpensive target and therefore desirably result in a low productioncost. ThO₂, TiO₂, AlN and TiN, on the other hand, are desirable in viewof the ease with which to fabricate the film.

Other desirable materials than those described above desirably have aheat conductivity larger than the substrate material and an absorptioncoefficient k smaller than 0.5.

A large heat conductivity can suppress the thermal damage to thesubstrate surface at the time of recording, and the jitter can be heldat a low level after 100 thousand overwrite cycles. Also, a small k canhold the reduction in modulation degree to a small value.

The materials and the replacement materials of the heat diffusion layer1 desirably represent 90% or more of all the atoms of each protectivelayer. Once the impurities other than the materials described abovereach 10 atomic % or more, the possible number of overwrite cycles isreduced to one half or less, or otherwise the rewrite characteristic isdeteriorated.

Changing the thickness of the heat diffusion layer used in thisembodiment, the jitter (σ/Tw) after 100 thousand overwrite cycles wasmeasured, and the following result was obtained. Also, the study made ofthe film-fabrication time for all the layers shows that since thesputter rate of the heat diffusion layer is low, the film-fabricationtime depends to a large measure on the thickness of the heat diffusionlayer. The film-fabrication time for the heat diffusion layer of 30 nmis assumed to be unity.

Heat diffusion Film-fabrication layer thickness (nm) Jitter (%) time  021 — 10 18 — 20 15 one time 30 15 one time 40 15 one time 50 — 1.2 times60 — 1.2 times

This indicates that the thickness of the heat diffusion layer isdesirably 10 to 50 nm, and more desirably 20 to 40 nm.

Also, the jitter increase is suppressed after a multiplicity ofoverwrite cycles not only for the disk having the structure shown inthis invention but also for the disk having the current structure andthe phase change disk having other heat diffusion layer.

(Reflective Layer)

The material Al—Cr used for the first reflective layer 6 in thisembodiment is desirably replaced by a material containing an Al alloy asa main component such as Al—Ti, Al—Ag or Al—Cu which can reduce thejitter at the time of overwrite operation.

This indicates that the characteristic at the time of a multiplicity ofoverwrite cycles is improved in the case where the content of theelements other than Al in the Al alloy is not less than 5 atomic % butnot more than 30 atomic %. A similar characteristic is obtained for anAl alloy other than those mentioned above.

As an alternative, a layer comprising any of the element units of Au,Ag, Cu, Ni, Fe, Co, Cr, Ti, Pd, Pt, W, Ta, Mo, Sb, Bi, Dy, Cd, Mn, Mgand V, or an alloy with any of these elements as a main component suchas an Au alloy, Ag alloy, Cu alloy, Pd alloy, Pt alloy, Sb—Bi, SUS orNi—Cr, or a combination of any of these alloys. In this way, the firstreflective layer is composed of a metal element, a metalloid element, analloy or a mixture thereof.

Among them, a material having a high reflectance such as Cu alloy, Alalloy or Au alloy increases the modulation degree and exhibits asuperior reproduction characteristic. The Ag alloy also has a similarcharacteristic. The contents of elements other than the main component,like the Al alloy, is set in the range of 5 atomic % to 30 atomic %inclusive, whereby the rewrite characteristic is improved further.

According to this embodiment, the materials of the second reflectivelayer replacing Al—Ti used in the second reflective layer 9 aredesirably Al—Ag, Al—Cu, Al—Cr or the like materials containing the Alalloy as a main component. Al is also usable.

From these facts, it has been found that in the case where the contentsof the elements other than Al in the Al alloy is in the range of 0.5atomic % to 4 atomic % inclusive, the characteristic after amultiplicity of overwrite cycles and the bit error rate are improved andthe improvement is further enhanced in the case where the contents arein the range of one atomic % to 2 atomic % inclusive. An Al alloy otherthan those mentioned above can also produce a similar characteristic.

Also, a layer comprising the element units of Au, Ag, Cu, Ni, Fe, Co,Cr, Ti, Pd, Pt, W, Ta, Mo, Sb, Bi, Dy, Cd, Mn, Mg or V, or an alloy withany of these elements as a main component such as an Au alloy, Ag alloy,Cu alloy, Pd alloy or Pt alloy, or an alloy of any of these alloys as amain component or an alloy of any combinations of these alloys. In thisway, the second reflective layer is composed of a metal element, ametalloid element or an alloy or a mixture thereof.

Among them, Cu, Al, Au, Cu alloy, Al alloy, Au alloy or the like whichhas a large heat conductivity tends to cool the disk rapidly andexhibits a superior rewrite characteristic. The Ag and Ag alloys havealso a similar characteristic. The content of elements other than themain component such as Cu, Au or Ag, like the Al alloy, is desirably inthe range of 0.5 atomic % to 4 atomic % inclusive, in which case thecharacteristic after a multiplicity of overwrite cycles and the biterror rate are improved, and the improvement is further enhanced whenthe contents are in the range of one atomic % to 2 atomic % inclusive.

Also, a study of the refractive index (n) and the extinction coefficient(k) of the materials of the first reflective layer and the secondreflective layer shows that n of the first reflective layer is largerthan n of the second reflective layer, and in the case where n of thefirst reflective layer is larger than n of the second reflective layerand k of the first reflective layer is smaller than k of the secondreflective layer, the increase of jitter after one hundred thousandoverwrite cycles can be suppressed within 3%.

The materials of the first reflective layer and the second reflectivelayer are desirably not less than 95% of the total number of atoms ofeach reflective layer. In the case where the impurities other than thematerials described above amount to 5 atomic % or more, the possiblenumber of overwrite cycles is reduced to one half or otherwise therewrite characteristic is deteriorated.

In the case where the second reflective layer is thinner than 30 nm, thestrength is so low and the heat diffusion is so small that the recordingfilm is liable to flow. Thus, the jitter after 100 thousand overwritecycles increases beyond 15%. The thickness of 40 nm can reduce thejitter to 15%. In the case where the thickness of the first reflectivelayer is larger than 100 nm or the thickness of the second reflectivelayer is larger than 200 nm, the time for fabricating the respectivereflective layer is lengthened to such an extent that the fabrication isdivided into two or more processes or two or more vacuum chambers areprovided for sputtering, thereby doubling the time required forfabrication. In the case where the thickness of the first reflectivelayer is not more than 5 nm, on the other hand, it is difficult to forma uniform film.

This indicates that the desirable thickness of the first reflectivelayer is not less than 5 nm but not more than 100 nm, and that of thesecond reflective layer is not less than 30 nm but not more than 200 nm.

(Combination of Materials of First Reflective Layer and SecondReflective Layer)

The materials described in this embodiment can be used for the firstreflective layer and the second reflective layer. By selecting acombination of the materials, however, it has been found that theincrease of jitter after 100 thousand overwrite cycles can be suppressedto not more than 3% for an improved rewrite characteristic. A preferablecombination is the first reflective layer of Al₉₄Cr₆ with the secondreflective layer of Al₉₈, Ti₁, the first reflective layer of Al₉₀Ti₁₀with the second reflective layer of Al₉₈Ti₂, the first reflective layerof Al₇₅Ti₂₅ with the second reflective layer of Al₉₉Ti₁, or the likecombinations in which the first reflective layer and the secondreflective layer contain the same main component element and the secondreflective layer contains more elements other than the main componentelement Al than the first reflective layer. The combination of Al—Tiwith Al—Ti, the combination of Al—Cr with Al—Cr or the like combinationcontaining an Al alloy such as Al—Ag or Al—Cu as a main component hasproduced a similar Characteristic. Following these materials, an Aualloy, Ag alloy, Cu alloy or a material of a similar composition hasexhibited an improved rewrite characteristic after a multiplicity ofoverwrite cycles.

(Substrate)

According to this embodiment, a polycarbonate substrate 1 having atracking groove directly formed in the surface thereof is used. In placeof this, polyolefin, epoxy, acrylic resin or a chemically reinforcedglass having the surface thereof formed with an ultraviolet settingresin layer may be used with equal effect.

The substrate having a tracking groove is defined as a substrate havinga groove at least λ/10n′ deep (n′: refractive index of substratematerial), where λ is the recording/reproduction wavelength, formed overthe whole or in a part of the substrate surface. The groove may beformed either continuously around the whole periphery or in a form splitmidway. It has been found that crosstalks are reduced desirably for thegroove depth of about λ/6n′. Further, in the case where the groove isdeeper than about λ/3n′, the cross erase is desirably reduced at thesacrifice of a lower yield of the process for forming the substrate.

Also, the groove width may be different at different places. Further, asubstrate of sample servo format lacking a groove, or a substrate ofother tracking systems or other formats will do. The substrate is eitherin a format capable of recording and reproduction in both the groove andthe land, or in a format capable of recording only in the groove or theland. The disk size is not limited to 12 cm but may assume other sizessuch as 13 cm, 3.5′ or 2.5′. The disk may be 1.2 mm, 0.8 mm thick orotherwise thick as well as 0.6 mm thick.

According to this embodiment, two disk members are fabricated by exactlythe same method, are these disk members are attached to each other withthe second reflective layers 9, 9′ thereof face to face through anadhesive layer. In place of the second disk member, however, a diskmember of another structure or a protective substrate may be used. Inthe case where the disk members used for attachment or the protectivesubstrate has a large transmittance in the ultraviolet wavelength area,the ultraviolet setting resin may be used for attachment. Other methodsmay also be used for attachment. In the disk member of a structurelacking the second reflective layer 9, an adhesive layer may be formedon the topmost layer.

In this embodiment, two disk members are prepared, and the first andsecond disk members are attached to each other with the secondreflective layers 9, 9′ thereof face to face through the adhesive layer10. If an ultraviolet setting resin is coated to the thickness of about10 μm on the second reflective layers 9, 9′ before attaching them andthey are attached to each other after the resin is set, then the errorrate can be reduced further.

Also, according to this embodiment, two disk members are prepared, andthe first and second disk members are attached to each other with thesecond reflective layers 9 thereof face to face through the adhesivelayer 10. Without so attaching, however, the ultraviolet setting resinabout 10 μm thick may be coated on the second reflective layer 9 of thefirst disk member.

In the case of a disk member lacking the second reflective layer 9, onthe other hand, the ultraviolet setting resin may be coated on thetopmost layer.

(Disk Structure)

The structures of the disks 1 to 39 described below other than thestructure described above in this embodiment also have the effect ofreducing the jitter by reducing the remanence due to the presence of anabsorption control layer. The materials, thickness, etc. of each layerare described in detail in the embodiments 1, 3, 4, 5. Also, the disk 3is described in detail in embodiment 3, the disk 4 in embodiment 6, andthe disk 24 in embodiment 5. Among these structures, in the case wherethe number of layers is 5 or 6 except for the substrate and the adhesivelayer, the apparatus used for producing a film is inexpensive and thewhole production cost can be reduced.

Disk 1: Substrate 1, heat diffusion layer 2, protective layer 3, lowersurface protect layer 4, recording film 5, upper surface protect layer6, absorption control layer 7, first reflective layer 8, adhesive layer10

Disk 2: Substrate 1, heat diffusion layer 2, protective layer 3, lowersurface protect layer 4, recording film 5, upper surface protect layer6, absorption control layer 7, second reflective layer 9, adhesive layer10

Disk 3: Substrate 1, heat diffusion layer 2, protective layer 3, lowersurface protect layer 4, recording film 5, upper surface protect layer6, absorption control layer 7, adhesive layer 10

Disk 4: Substrate 1, heat diffusion layer 2, protective layer 3, lowersurface protect layer 4, recording film 5, absorption control layer 7,first reflective layer 8, second reflective layer 9, adhesive layer 10

Disk 5: Substrate 1, heat diffusion layer 2, protective layer 3, lowersurface protect layer 4, recording film 5, absorption control layer 7,first reflective layer 8, adhesive layer 10

Disk 6: Substrate 1, heat diffusion layer 2, protective layer 3, lowersurface protect layer 4, recording film 5, absorption control layer 7,second reflective layer 9, adhesive layer 10

Disk 7: Substrate 1, heat diffusion layer 2, protective layer 3, lowersurface protect layer 4, recording film 5, absorption control layer 7,adhesive layer 10

Disk 8: Substrate 1, heat diffusion layer 2, protective layer 3,recording film 5, upper surface protect layer 6, absorption controllayer 7, first reflective layer 8, second reflective layer 9, adhesivelayer 10

Disk 9: Substrate 1, heat diffusion layer 2, protective layer 3,recording film 5, upper surface protect layer 6, absorption controllayer 7, first reflective layer 8, adhesive layer 10

Disk 10: Substrate 1, heat diffusion layer 2, protective layer 3,recording film 5, upper surface protect layer 6, absorption controllayer 7, second reflective layer 9, adhesive layer 10

Disk 11: Substrate 1, heat diffusion layer 2, protective layer 3,recording film 5, upper surface protect layer 6, absorption controllayer 7, adhesive layer 10

Disk 12: Substrate 1, heat diffusion layer 2, protective layer 3,recording film 5, absorption control layer 7, first reflective layer 8,second reflective layer 9, adhesive layer 10

Disk 13: Substrate 1, heat diffusion layer 2, protective layer 3,recording film 5, absorption control layer 7, first reflective layer 8,adhesive layer 10

Disk 14: Substrate 1, heat diffusion layer 2, protective layer 3,recording film 5, absorption control layer 7, first reflective layer 8,second reflective layer 9, adhesive layer 10

Disk 15: Substrate 1, heat diffusion layer 2, protective layer 3,recording film 5, absorption control layer 7, adhesive layer 10

Disk 16: Substrate 1, protective layer 3, lower surface protect layer 4,recording film 5, upper surface protect layer 6, absorption controllayer 7, first reflective layer 8, second reflective layer 9, adhesivelayer 10

Disk 17: Substrate 1, protective layer 3, lower surface protect layer 4,recording film 5, upper surface protect layer 6, absorption controllayer 7, first reflective layer 8, adhesive layer 10

Disk 18: Substrate 1, protective layer 3, lower surface protect layer 4,recording film 5, upper surface protect layer 6, absorption controllayer 7, second reflective layer 9, adhesive layer 10

Disk 19: Substrate 1, protective layer 3, lower surface protect layer 4,recording film 5, upper surface protect layer 6, absorption controllayer 7, adhesive layer 10

Disk 20: Substrate 1, heat diffusion layer 2, lower surface protectlayer 4, recording film 5, upper surface protect layer 6, absorptioncontrol layer 7, first reflective layer 8, second reflective layer 9,adhesive layer 10

Disk 21: Substrate 1, heat diffusion layer 2, lower surface protectlayer 4, recording film 5, upper surface protect layer 6, absorptioncontrol layer 7, first reflective layer 8, adhesive layer 10

Disk 22: Substrate 1, heat diffusion layer 2, lower surface protectlayer 4, recording film 5, upper surface protect layer 6, absorptioncontrol layer 7, second reflective layer 9, adhesive layer 10

Disk 23: Substrate 1, heat diffusion layer 2, lower surface protectlayer 4, recording film 5, upper surface protect layer 6, absorptioncontrol layer 7, adhesive layer 10

Disk 24: Substrate 1, protective layer 3, lower surface protect layer 4,recording film 5, upper surface protect layer 6, absorption controllayer 7, first reflective layer 8, second reflective layer 9, adhesivelayer 10

Disk 25: Substrate 1, protective layer 3, lower surface protect layer 4,recording film 5, upper surface protect layer 6, absorption controllayer 7, first reflective layer 8, adhesive layer 10

Disk 26: Substrate 1, protective layer 3, lower surface protect layer 4,recording film 5, upper surface protect layer 6, absorption controllayer 7, second reflective layer 9, adhesive layer 10

Disk 27: Substrate 1, protective layer 3, lower surface protect layer 4,recording film 5, upper surface protect layer 6, absorption controllayer 7, adhesive layer 10

Disk 28: Substrate 1, protective layer 3, lower surface protect layer 4,recording film 5, absorption control layer 7, first reflective layer 8,second reflective layer 9, adhesive layer 10

Disk 29: Substrate 1, protective layer 3, lower surface protect layer 4,recording film 5, absorption control layer 7, first reflective layer 8,second reflective layer 9, adhesive layer 10

Disk 30: Substrate 1, protective layer 3, lower surface protect layer 4,recording film 5, absorption control layer 7, first reflective layer 8,adhesive layer 10

Disk 31: Substrate 1, protective layer 3, lower surface protect layer 4,recording film 5, absorption control layer 7, adhesive layer 10

Disk 32: Substrate 1, protective layer 3, recording film 5, uppersurface protect layer 6, absorption control layer 7, first reflectivelayer 8, second reflective layer 9, adhesive layer 10

Disk 33: Substrate 1, protective layer 3, recording film 5, uppersurface protect layer 6, absorption control layer 7, second reflectivelayer 9, adhesive layer 10

Disk 34: Substrate 1, protective layer 3, recording film 5, uppersurface protect layer 6, absorption control layer 7, first reflectivelayer 8, adhesive layer 10

Disk 35: Substrate 1, protective layer 3, recording film 5, uppersurface protect layer 6, absorption control layer 7, adhesive layer 10

Disk 36: Substrate 1, protective layer 3, recording film 5, absorptioncontrol layer 7, first reflective layer 8, second reflective layer 9,adhesive layer 10

Disk 37: Substrate 1, protective layer 3, recording film 5, absorptioncontrol layer 7, second reflective layer 9, adhesive layer 10

Disk 38: Substrate 1, protective layer 3, recording film 5, absorptioncontrol layer 7, first reflective layer 8, adhesive layer 10

Disk 39: Substrate 1, protective layer 3, recording film 5, absorptioncontrol layer 7, adhesive layer 10

(Thickness and Material of Each Layer)

The recording/reproduction characteristic is improved simply byemploying a preferable range of the thickness and the material of eachlayer independently. By combining preferable ranges, however, the effectis further enhanced.

(2) Embodiment 2

(Configuration and Fabrication Method)

For clarifying the effects of the absorption control layer, a disk-typeinformation recording medium of a structure lacking the absorptioncontrol layer was prepared. FIG. 2 is a sectional view showing thestructure of this medium. This medium is fabricated in the followingway.

First, a heat diffusion layer 2 of Al₂O₃ film about 30 nm thick wasformed on a polycarbonate substrate 12 cm in diameter, 0.6 mm thick andhaving a tracking groove in the surface thereof. Then, a protective film3 of ZnS about 45 nm thick, a lower surface protect layer 4 of SiO₂ filmabout 5 nm thick, a recording film 5 of Ge₁₄Sb₂₉Te₅₇ about 15 nm thick,an upper surface protect layer 6 of SiO₂ film about 15 nm thick and areflective layer 11 of Al₉₈Ti₂ about 200 nm thick were sequentiallyformed, and two disk members prepared in similar manner are attached toeach other thereby to produce a disk-type information recording mediumshown in FIG. 2.

(Recording/Reproduction Characteristic)

The initial crystallization, recording, erasure and reproduction areperformed in the same manner as in embodiment 1. With an informationrecording medium having a configuration lacking an absorption controllayer according to this embodiment, the overwrite operation, ifperformed when repeating the recording and reproduction, increases thejitter considerably as compared with the information recording medium asdescribed in embodiment 1, as shown in FIG. 4.

This indicates that the absence of the absorption control layerincreases the jitter after 10 overwrite cycles. This increased jitterprobably stems from the fact that the absorption coefficient cannot besufficiently controlled due to the absorption ratio (Ac/Aa) as small asabout 0.9, thereby causing a remanence. The remanence is more liable tooccur when the linear speed is high.

(3) Embodiment 3

An information recording medium was prepared as follows in the samemanner as in embodiment 1 except that the first reflective layer 8 ofembodiment 1 is eliminated. Specifically, in the information recordingmedium according to embodiment 3, a heat diffusion layer 2 of Al₂O₃ filmabout 30 nm thick was formed on a polycarbonate substrate 1 having adiameter of 12 cm and a thickness of 0.6 mm with a tracking grooveformed in the surface thereof. Then, a protective film of ZnS film about45 nm thick, a lower surface protect layer 4 of SiO₂ film about 5 nmthick, a recording film 5 of Ge₁₄Sb₂₉Te₅₇ about 15 nm thick, an uppersurface protect layer 6 of SiO₂ film about 15 nm thick, an absorptioncontrol layer 7 of Mo₈₀(SiO₂)₂₀ film about 18 nm thick, and a secondreflective layer 9 of Al₉₈Ti₂ film about 180 nm thick, were sequentiallyformed. Two disk members prepared in a similar manner were attached toeach other thereby to produce a disk-type information recording mediumas shown in FIG. 5.

With the disk according to this embodiment, the first reflective layeris eliminated and therefore the time for preparing the disk could beshortened by the time corresponding to one layer as compared with thedisk of embodiment 1.

(Recording/Reproduction Characteristic)

The recording/reproduction characteristic was studied in the same manneras in embodiment 1. As a result, the same effect of the absorptioncontrol layer was obtained as in embodiment 1.

In addition, in view of the fact that the second reflective layer havinga large heat conductivity is in direct contact with the absorptioncontrol layer, the heat is easily lost and the cross erase could bereduced. The cross erase is a phenomenon in which assuming that therecording is made in a track (T2) adjacent to a track (T1) where themark is recorded, the signal amount is reduced due to the disappearanceof the recording mark written in the track (T2) by the heat generated atthe time of recording. On the other hand, the jitter at the front edgeafter 10 overwrite cycles increased by 3%.

The matter not described in this embodiment is similar to thecorresponding matter in embodiment 1.

(4) Embodiment 4

With the exception that the composition of the recording films 5, 5′ ofembodiment 1 was changed in the following manner, an informationrecording medium having the following composition of the recording filmwas prepared in the same manner as in embodiment 1. The initialcrystallization, recording, erasure and reproduction were carried out inthe same manner as in embodiment 1.

(Composition of Recording Film)

The composition of the recording films 5, 5′ used in this embodiment arechanged along the lines connecting GeTe and Sb₂Te₃ in a triangulardiagram, and the jitter (σ/Tw) was measured after ten overwrite cycleswith the following result.

Recording film Jitter at Jitter at composition front edge (%) trailingedge (%) Ge₈Sb₃₄Te₅₈ 23 — Ge₁₀Sb₃₂Te₅₈ 18 — Ge₁₂Sb₂₇Te₅₆ 15 14Ge₁₄Sb₂₉Te₅₇ 14 14 Ge₂₄Sb₂₁Te₅₅ 14 15 Ge₂₆Sb₁₉Te₅₅ — 18Ge₂₈Sb_(17.5)Te_(54.5) — 22

This indicates that with the increase in Ge amount, the jitter at thefront edge is reduced while the jitter at the trailing edge isincreased. Thus, the Ge amount exhibiting a superior jittercharacteristic is in the range of not less than 10 atomic % but not morethan 26 atomic %, and a better characteristic is exhibited in the rangeof not less than 12 atomic % but not more than 24 atomic %.

Then, while maintaining a constant Te amount in the composition of therecording film and changing the Te and Sb amounts, the jitter (σ/Tw)after 10 overwrite cycles was measured with the following result.

Recording film Jitter at Jitter at composition front edge (%) trailingedge (%) Ge₂₇Sb₁₆Te₅₇ — 22 Ge₂₅Sb₁₈Te₅₇ — 18 Ge₂₃Sb₂₀Te₅₇ 15 15Ge₁₄Sb₂₉Te₅₇ 14 14 Ge₁₂Sb₃₁Te₅₇ 14 15 Ge₁₀Sb₃₃Te₅₇ 18 — Ge₈Sb₃₅Te₅₇ 23 —

This indicates that with the increase in Sb amount, the jitter at thefront edge is increased while the jitter at the trailing edge isreduced. Thus, the Sb amount exhibiting a superior jitter characteristicis in the range of not less than 18 atomic % but not more than 33 atomic%, and a better characteristic is exhibited in the range of not lessthan 20 atomic % but not more than 31 atomic %.

Then, while maintaining a constant Sb amount in the composition of therecording films 5, 5′ and changing the Te and Ge amounts, the jitter(σ/T-w) after 10 overwrite cycles was measured with the followingresult.

Recording film composition Jitter at trailing edge (%) Ge₉Sb₂₉Te₆₂ 23Ge₁₁Sb₂₉Te₆₀ 18 Ge₁₃Sb₂₉Te₅₈ 15 Ge₁₄Sb₂₉Te₅₇ 14 Ge₁₇Sb₂₉Te₅₄ 15Ge₁₉Sb₂₉Te₅₂ 18 Ge₂₀Sb₂₉Te₅₁ 22

This indicates that the jitter at the trailing edge increases regardlessof whether the Te amount is increased or decreased. Thus, the Te amountassociated with a superior jitter characteristic is in the range of notless than 52 atomic % but not more than 60 atomic %, and a better jittercharacteristic is exhibited in the range of not less than 54 atomic %but not more than 58 atomic %.

In this embodiment, by adding Ag to the recording film to form anAg—Ge—Sb—Te recording film, it has been found that as compared with theGe—Sb—Te, the number of a multiplicity of overwrite cycles at which thejitter at the front edge increases by 5% or more increases twice. Inview of this, while maintaining constant Sb and Te amounts in thecomposition of the recording films 5, 5′ and changing the Ge and Agamounts, the jitter (σ/Tw) after 5 overwrite cycles was measured withthe following result. Also, the number of overwrite cycles at which thejitter increases at least 5% was studied.

Recording film Number of rewrite composition Jitter (%) operationsGe₁₄Sb₂₉Te₅₇ 14 One time Ag₁Ge₁₃Sb₂₉Te₅₇ 14 1.5 times Ag₂Ge₁₂Sb₂₉Te₅₇ 15Twice Ag₄Ge₁₀Sb₂₉Te₅₇ 15 Twice Ag₆Ge₈Sb₂₉Te₅₇ 20 — Ag₈Ge₆Sb₂₉Te₅₇ 24 —

This indicates that addition of a small amount of Ag improves the numberof possible overwrite cycles. With the increase in Ag amount, however,it has been found that the jitter also increases. Thus, the Ag amountassociated with a superior jitter characteristic is in the range of notmore than 6 atomic % and a better jitter characteristic is exhibited inthe range of not more than 4 atomic %.

From the foregoing description, it is seen that in the case where thecomposition of the recording film is expressed asGe_(x-w)Sb_(y)Te_(z)M_(w)(x+y+z=1), a superior characteristic isexhibited in the range of 0.10≦x≦0.26, 0.18≦y≦0.33, 0.52≦z≦0.60, 0≦w≦0.06. A better characteristic is exhibited in the range of0.12≦x≦0.24, 0.20≦y≦0.31, 0.54≦z≦0.58, 0≦w≦0.04.

Further, when the Ge amount reaches 20 atomic % or more in this range,the read light endurance is improved by 1.5 times. The read lightendurance is determined by comparison with the power of the read lightat which the recording signal is reduced by at least 2 dB during afive-minute reproduction. Also, in the case where the Ge amount is notmore than 17 atomic % and the linear speed is high, the extinction ratiois large. Even with the linear speed of 12 m/s, the extinction ratio wasas superior as not less than 30 dB.

Elements added to the recording film in place of Ag include Na, Mg, Al,P, S, Cl, L, Ca, Sc, Zn, Ga, As, Se, Br, Rb, Sr, Y, Zr, Nb, Ru, Rh, Cd,In, Sn, I, Cs, Ba, La, Hf, Ta, Re, Os, Ir, Hg, Tl, Pb, Th, U, Cr, W, Mo,Pt, Co, Ni, Pd, Si, Au, Cu, V, Mn, Fe, Ti and Bi. It has been found thateven when Ag is replaced by at least one of these elements, the jitteris not easily increased after many overwrite cycles.

Among these elements, the addition of Ag increases the recordingsensitivity by 10% as compared with Ge—Sb—Te, the addition of at leastone of Cr, W and Mo at least triples the number of a multiplicity ofoverwrite cycles at which the jitter increases 5% or more, and theaddition of at least one of Pt, Co and Pd increases the crystallizationtemperature by 50° C. or more as compared with Ge—Sb—Te.

Also, it has been found that when the impurities elements contained inthe recording film exceeds 2 atomic % of the recording film components,the jitter at the front edge or the trailing edge after 10 overwritecycles exceeds 15%. It has further been found that in the case where theimpurities elements exceeds 5 atomic %, the jitter increases to at least18%. Thus, a desirable content of the impurities elements in therecording film is not more than 5 atomic % of the recording filmcomponents at which the deterioration of the rewrite characteristic isnot reduced so much. The content of not more than 2 atomic % is moredesirable.

While changing the thickness of the recording films 5, 5′ used in thisembodiment, the jitter (σ/Tw) after 10 overwrite cycles and 100 thousandoverwrite cycles was measured and the following result was obtained. Foreach recording film thickness (nm), the value of jitter (%) at the frontedge or the trailing edge, whichever is worse, is shown for thecharacteristic after 10 overwrite cycles, and the value of jitter (%) atthe front edge is shown for the characteristic after 100 thousandoverwrite cycles.

Recording film Jitter after Jitter after thickness 10 rewrites 10⁵rewrites  8 23 — 10 20 — 13 15 15 18 14 15 20 15 15 30 — 20 40 — 25

This indicates that with the decrease in the thickness of the recordingfilm, the jitter after ten overwrite cycles is increased due to the flowor segregation of the recording film, while with the increase in thethickness of the recording film, the jitter after 100 thousand overwritecycles increases. It is thus seen that the desirable thickness of therecording film is in the range of not less than 10 nm but not more than30 nm, or more desirably in the range of not less than 13 nm but notmore than 20 nm.

Though somewhat time consuming for fabricating the recording film, ithas been found that mixing nitrogen with the sputtering gas in theinitial or last stage of the recording film fabrication, using a targetcontaining a slight amount of nitrogen in the composition of therecording film or otherwise adding nitrogen in the neighborhood of theboundary between the recording film and other layers, the adhesiveamount is increased for an improved characteristic.

The matters not described in this embodiment are similar to thecorresponding matters in embodiments 1 and 3.

(5) Embodiment 5

(Configuration and Fabrication Method)

The following information recording medium was fabricated in a mannersimilar to embodiment 1 except that in this embodiment, the heatdiffusion layer 2 is not included.

Specifically, a protective layer 3 of ZnS film about 45 nm thick, alower surface protect layer 4 of SiO₂ film about 5 nm thick, a recordingfilm 5 of Ge₁₄Sb₂₉Te₅₇ about 15 nm thick, an upper surface protect layer6 of SiO₂ film about 15 nm thick, an absorption control layer 7 ofMo₈₀(SiO₂)₂₀ film about 18 nm thick, a first reflective layer 8 ofAl₈₉Ti₁₁ film about 20 nm thick and a second reflective film 9 ofAl₉₈Ti₂ about 180 nm were formed in that order sequentially. Two diskmembers fabricated in the same manner are attached to each other therebyto produce a disk-type information recording medium shown in FIG. 6.

Also, with the disk according to this embodiment, in which the heatdiffusion layer is not included, the disk fabrication time could beshortened by a length equivalent to one layer as compared with the diskof embodiment 1. The heat diffusion layer takes a long time to form, andtherefore the absence of the heat diffusion layer could reduce the totalfabrication time by about one fourth.

(Recording/Reproduction Characteristic)

The recording/reproduction characteristic was studied by the same methodas in embodiment 1. As a result, the effect of the absorption controllayer similar to that of embodiment 1 was obtained.

In addition, the temperature rise of the substrate which has beensuppressed by the presence of the heat diffusion layer increases thejitter after 100 thousand overwrite cycles to 18% as compared with thecase having the heat diffusion layer. Substantially no difference isobserved for ten thousand or less overwrite cycles.

Those matters not described in this embodiment are similar to thecorresponding matters in embodiments 1, 3 and 4.

(6) Embodiment 6

(Configuration and Fabrication Method)

An information recording medium was fabricated as follows in the samemanner as in embodiment 1 except that the first reflective layer 8 andthe second reflective layer 9 were not included.

Specifically, in the information recording medium according toembodiment 6, a heat diffusion layer 2 of Al₂O₃ film, a protective layer3 of ZnS film about 45 nm thick, a lower surface protect layer 4 of SiO₂film about 5 nm thick, a recording film of Ge₁₄Sb₂₉Te₅₇ about 15 nmthick, an upper surface protect layer 6 of SiO₂ film about 15 nm thickand an absorption control layer 7 of Mo₈₀ (SiO₂)₂₀ film about 18 nmthick, were formed sequentially in that order on a polycarbonatesubstrate 1 having a diameter of 12 cm and a thickness of 0.6 mm withthe surface thereof formed with a tracking groove. Two disk membersprepared in the same manner are attached to each other thereby toproduce a disk-type information recording medium shown in FIG. 7.

Also, the absence of the heat diffusion layer in the disk according tothis embodiment could reduce the disk fabrication time by a timecorresponding to two layers as compared with the time required inembodiment 1. As a result, the total production time could be shortenedconsiderably to about two thirds.

(Recording/Reproduction Characteristic)

The recording/reproduction characteristic was studied in the same manneras in embodiment 1, and it was found that as in embodiment 1, the jittervalue can be reduced as compared with a disk structure having noabsorption control layer.

(Thickness of Absorption Control Layer)

In the case where the reflective layer is lacking as in this embodiment,it has been found that the thickness of the absorption control layer notless than 50 nm is sufficient for maintaining a high strength. Also, thethickness of not more than 200 nm is desirable for shortening thefabrication time.

The matters not described with this embodiment are similar to thecorresponding matters in embodiments 1 and 3 to 4.

As described above, according to the present invention, there isprovided an information recording medium comprising an informationrecording thin film formed on a substrate as a recording layer forrecording and/or reproducing the information by the change of atomicarrangement caused by radiation of light, and at least one protectivelayer, characterized in that the protective layer and the recordinglayer are formed in that order from the light incidence side, the mediumfurther comprising at least one absorption control layer, whereby thejitter (σ/Tw) at the time of overwrite operation can be reduced ascompared with the information recording medium lacking the absorptioncontrol layer. This is by reason of the fact that the remanence can bereduced due to the presence of the absorption control layer.

The absorption control layer is composed of a metal element or anunsaturated metal oxide and a dielectric material, and has the functionof controlling the absorption coefficient Ac of the recording film incrystal state to not less than the absorption coefficient Aa of therecording medium in amorphous state.

Further, in the case where the thickness of the absorption control layeris set in the range of not less than 5 nm but not more than 50 nm, thejitter after 10 overwrite cycles can be reduced with an increasedmodulation degree.

The surface protect layer is formed in the boundary of the recordingmedium and has the effect of increasing the crystallization rate of therecording film and improving the extinction characteristic thereof. Theprotective layer is formed between the recording film and and substrate,and has the effect of protecting the recording film. The heat diffusionlayer, which is made of a material having a larger heat conductivitythan the substrate and formed directly on the substrate, has the effectof preventing the temperature increase of the substrate at the time of amultiplicity of overwrite cycles thereby to reduce the increase injitter. The first reflective layer has the effect of preventing theincrease in jitter at the front edge, and the second reflective layer,due to its large heat conductivity, has the effect of reducing theincrease in jitter after a multiplicity of overwrite cycles. Further,both the first reflective layer and the second reflective layer preventthe light from being transmitted through the medium and thus improve therecording sensitivity.

What is claimed is:
 1. An information recording medium comprising: asubstrate; a recording layer on which information is recorded inaccordance with a change of atomic arrangement caused by irradiation oflight; a first reflective layer including an Al alloy as a maincomponent; and a second reflective layer including a Ag alloy as a maincomponent formed on the first reflective layer.
 2. An informationrecording medium according to claim 1, wherein an absorption controllayer is formed between the recording layer and the first reflectivelayer.
 3. An information recording medium according to claim 2, whereinthe absorption control layer has a refraction index of not less than 1.2and not more than 6 and an extinction of not less than 0.5 and not morethan 3.3.
 4. An information recording medium comprising: a substrate; arecording layer on which information is recorded in accordance with achange of atomic arrangement caused by irradiation of light; a firstreflective layer including an Al alloy and elements other than Al beingset in a range of 5 atomic % to 30 atomic %; and a second reflectivelayer including an Al alloy and elements other than Al being set in arange of 0.5 atomic % to 4 atomic % formed on the first reflectivelayer.
 5. An information recording medium according to claim 4, whereinan absorption control layer is formed between the recording layer andthe first reflective layer.
 6. An information recording medium accordingto claim 5, wherein the absorption control layer has a refraction indexof not less than 1.2 and not more than 6 and an extinction of not lessthan 0.5 and not more than 3.3.